1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive sheet for protecting a semiconductor wafer, which causes little warpage in a semiconductor wafer after a semiconductor wafer has been ground to be extremely thin or after a large diameter wafer has been ground.
2. Description of the Related Art
In recent years, along with the miniaturization of various electronic equipments and the popularization of an IC card, electronic parts such as a semiconductor wafer are desired to be further thinned. For this reason, the conventional semiconductor wafer having had the thickness of approximately 350 μm is needed to be thinned to approximately 30 μm or less. In addition, in order to enhance the productivity, it is investigated to further increase the diameter of the wafer.
Usually, in a process of manufacturing the semiconductor wafer, after a circuit pattern has been formed on the surface of the wafer, the rear face of the wafer is ground with a grinder or the like until the thickness of the wafer reaches a predetermined thickness. On this occasion, a pressure-sensitive adhesive sheet is generally affixed onto the surface of the wafer and then the rear face is ground, for the purpose of protecting the surface of the wafer. In addition, after the wafer has been worked into a thin type, the wafer is occasionally transported to a next step in a state of having the pressure-sensitive adhesive sheet affixed on its surface.
However, when the rear face has been ground to make the wafer extremely thin in such a state that the surface of the wafer has been protected by the pressure-sensitive adhesive sheet, the wafer which has been ground tends to easily cause warpage therein. The wafer which has caused the warpage therein has a problem of causing cracks therein while the wafer is transported and while the pressure-sensitive adhesive sheet is peeled from the wafer. This is considered to be because when the residual stress of the pressure-sensitive adhesive sheet exceeds the strength of the wafer as a result of having ground the rear face of the wafer having the pressure-sensitive adhesive sheet affixed thereon, the wafer is warped by such a force as to cancel the residual stress.
This warpage of the wafer which has been ground is considered to occur largely due to the influence of the residual stress remaining in the pressure-sensitive adhesive sheet. It is considered that this residual stress in the pressure-sensitive adhesive sheet constituted by a substrate and a tackiness agent is mainly formed in a step of coating the substrate with the tackiness agent or a manufacturing step of affixing a tackiness agent layer to the substrate and a step of sticking the pressure-sensitive adhesive sheet to the wafer, and when the wafer having the pressure-sensitive adhesive sheet affixed thereon in which the residual stress exists is ground to become extremely thin, the residual stress in the pressure-sensitive adhesive sheet exceeds the strength of the wafer and causes the warpage in the wafer due to such a force as to cancel the residual stress. In addition, therefore, in order to reduce this residual stress, various improvements have been made on the constitution of the pressure-sensitive adhesive sheet, and such a constitution as not to cause the residual stress has been proposed. For instance, Japanese Patent Laid-Open No. 2000-212524 proposes a pressure-sensitive adhesive sheet for protecting a semiconductor wafer, which is constituted by a substrate film and a tackiness agent layer and in which a modulus of tensile elasticity of the substrate film is 0.6 GPa.
In addition, Japanese Patent Laid-Open No. 2000-150432 proposes a pressure-sensitive adhesive sheet for use in working a semiconductor wafer, which is formed of a substrate and a tackiness agent layer formed on the substrate and in which the percentage of stress relaxation is 40% or more after 1 minute in an extension of 10% in a tensile test of the pressure-sensitive adhesive sheet.
The pressure-sensitive adhesive sheet to be affixed on the surface of the semiconductor wafer is generally formed by a constitution of a substrate layer and a tackiness agent layer. Such a pressure-sensitive adhesive sheet is manufactured by directly coating the substrate with the tackiness agent and affixing the substrate onto a separator or coating the separator with the tackiness agent and affixing the separator onto the substrate, in the manufacturing step. However, on this occasion, the substrate and the separator need to be stretched by some extent of tension so as not to hang slack, and accordingly a stress unavoidably occurs when the substrate and the separator are affixed to each other.
The substrate is also used for the purpose of enhancing supporting properties so that the pressure-sensitive adhesive sheet supports the semiconductor wafer to enhance the handleability.
When the pressure-sensitive adhesive sheet is affixed onto the surface of the wafer, the wafer is mounted on an affixing table so that the surface of the wafer faces upward, and the pressure-sensitive adhesive sheet is supplied thereon in such a state that the tackiness agent layer faces downward, while being pulled along the direction to be affixed so as not to hang slack, by using an affixing machine. Thus, the pressure-sensitive adhesive sheet is affixed to the wafer, by facing the tackiness agent layer of the pressure-sensitive adhesive sheet to the surface of the wafer, and sequentially crimping the substrate side of the pressure-sensitive adhesive sheet to the wafer along the direction to be affixed, with pressing means such as a crimping roll.
At this time as well, a force of pulling the pressure-sensitive adhesive sheet along the direction to be affixed and a force of crimping the pressure-sensitive adhesive sheet to the wafer are applied to the pressure-sensitive adhesive sheet, and accordingly when the pressure-sensitive adhesive sheet is affixed to the wafer, these forces remain in the pressure-sensitive adhesive sheet in a form of a residual stress.
In fact, various characteristics of these pressure-sensitive adhesive sheets as described in the above described Patent Documents are not necessarily optimal as one which suppresses the warpage of the wafer that has been ground when the semiconductor wafer is ground to be extremely thin or when a large diameter wafer is ground. For this reason, it has been desired to provide a pressure-sensitive adhesive sheet for protecting the semiconductor wafer, which can more effectively suppress the warpage of the wafer that has been ground.
Along with the recent tendency of extremely thinning the thickness of the ground wafer, it has also been desired that the wafer does not cause cracks in the wafer and chipping in the edge portion of the wafer due to the stress during the grinding operation. Though the pressure-sensitive adhesive sheet must be peeled from the wafer after the grinding operation, it has also been desired at this time that the tackiness agent does not remain on a circuit pattern on the surface of the wafer, and that there is no contamination at a molecular level on the surface of the wafer, which originates in the pressure-sensitive adhesive sheet, the semiconductor wafer and the like.
In addition, when the pressure-sensitive adhesive sheet having two or more layers including a substrate is used for fixing the wafer during a dicing operation, a so-called lump occurs that is a fine lump of the tackiness agent layer, which attaches to a blade and the pressure-sensitive adhesive sheet due to the change of a force applied to the blade and the like occurring in the interface between the two or more layers, because these layers have different elastic modulus from each other. The lump having attached to the blade or the pressure-sensitive adhesive sheet attaches to the wafer or the pressure-sensitive adhesive sheet in the subsequent step, and occasionally has made the cutting thereof difficult or has caused a crack in the wafer.
Furthermore, the pressure-sensitive adhesive sheet has caused a warpage of the semiconductor wafer and the like, and water and the like have occasionally intruded between the semiconductor wafer and the pressure-sensitive adhesive sheet when the semiconductor wafer has been washed with water or the like. For this reason, it has been required that the pressure-sensitive adhesive sheet shows stress relaxation properties, the tackiness agent layer of the pressure-sensitive adhesive sheet can sufficiently follow unevenness provided on the surface of the wafer, and lifting does not occur in the pressure-sensitive adhesive sheet by a shearing force in a cutting operation.
An object of the present invention is to provide a pressure-sensitive adhesive sheet for a semiconductor wafer, which does not cause warpage in the semiconductor wafer, even when the semiconductor wafer is ground to be extremely thin or a large diameter wafer is ground, is excellent in followability to a pattern, does not cause lifting from the pattern due to elapsing time, has adequate stress dispersibility in a grinding operation, suppresses the crack in a wafer and chipping in a wafer edge, does not cause peeling in the interlayer in a peeling operation, does not leave a residue of the tackiness agent on the surface of the wafer, and besides, does not produce a so-called lump made from the tackiness agent in a cutting operation.